1. Technical Field
The present invention relates to color image forming apparatuses such as a copying machine, a facsimile, and a printer and color image forming methods, and, more particularly to a color image forming apparatus and a color image forming method for forming a thin layer of a high-viscosity and high-density liquid developer on a developer carrying member in liquid developing means, bringing the liquid developer layer on the developer carrying member into contact with the surface of an image supporting member, and visualizing an electrostatic latent image on the image supporting member formed by electrophotography, electrostatic recording, ion flow, or the like.
2. Related Art
An image forming system that forms an image using a liquid developer has advantages that, for example, a fine toner in a sub-micron size can be used, a high-definition image quality can be realized, and a sufficient image density can be obtained with a small quantity of toner. These advantages cannot be realized by an image forming system that forms an image using a powder toner.
As the liquid developer, for example, there is known a liquid developer obtained by dispersing a toner in a volatile carrier liquid (JP-A-9-26704). In this patent document, it is described that a driving member is idled before a development operation to bring the driving member into a preparatory driving state in order to prevent occurrence of a coarse toner and occurrence of the adhesion and the like of the driving member due to vaporization of the carrier liquid. However, as long as the volatile carrier liquid is used, even if the driving member is brought into the preparatory driving state, it is difficult to prevent, for example, the adhesion to the surface of a transfer belt and the surface of a cleaning member in addition to the adhesion of the driving member.
When a nonvolatile high-viscosity and high-density liquid developer is used, the problems in the volatile carrier liquid can be prevented. However, as described in JP-A-2001-75365, the accumulation of the liquid developer and the swell of developer particle components occur in a developing unit. As in JP-9-26704, it is described that the developing unit is pre-driven before a development operation to solve the problems and normally recover a developing ability.
However, it has been found that, when a liquid developer that has a viscosity characteristic dependent on a shearing force and is obtained by dispersing a basic processed pigment in a vegetable oil according to an acid-base interaction is used as the nonvolatile liquid developer, even if a shearing force of agitation or the like is applied to the liquid developer to stabilize the viscosity in the liquid developer, stabilization time is different in respective colors and the viscosity in the liquid developer is different in the respective colors assumingly because the correlation between the pigment and a dispersant affects the stabilization. In such a liquid developer, since the use of the dispersant substantially affects an electric resistance in the liquid developer, when the electric resistance is set to the same degree in the respective colors as a premise, the difference in the viscosity is inevitable. When liquid developers of the respective colors are used for printing in a state in which the viscosity is unstable, the thickness of the liquid developers is not stable. Therefore, at an initial stage, a film is formed thick on a developing roller, an image section has density different from the original density, and thin lines are broken. Even if the viscosity is stabilized, the viscosity may be different in the respective colors. Therefore, uniform toner thin layers of the respective colors cannot be formed on respective developing rollers and it is difficult to adjust a color balance when color image formation is performed.